Microporous coordination polymers (MCPs) are poised to make a commercial impact on sorption technologies. Uniquely high performance has been demonstrated for gas storage,1 separations,2 CO2 capture,3 and catalysis.4 When considering that widely available activated carbons and zeolites can come close to reaching these performance levels at a cost of dollars per pound of sorbent and are already broadly deployed, the difficulty associated with deploying MCPs as drop in replacements for these established sorbents becomes obvious.
Advancing the promise of MCPs into impact is hampered by a number of synthetic issues and complexities associated with material activation. The majority of ligands employed thus far are impractical from a cost standpoint. Significant advances have been made by leveraging inexpensive starting materials through mixed linker MCPs5 and complexity generating schemes such as post synthetic modification.6 These new methods have the potential to control starting material costs sufficiently to make industrial synthesis possible; however, the material activation regimen remains problematic.
Often the solvent of synthesis is exchanged multiple times over several days and with multiple solvent types to facilitate guest removal and evacuation. In some cases this process must include supercritical CO2 (SC—CO2) drying in order to maintain porosity. Therefore even controlling the cost of material synthesis is insufficient without a means of economically activating the sorbent.
Some MCPs are unstable upon guest removal and undergo pore collapse. Hupp and co-workers proposed that high surface areas and permanent porosity for this kind of MCP could be obtained through activation by SC—CO2.7a Summarizing the general procedure for batch SC—CO2 activation as typically practiced,7a as-synthesized crystals are washed with N,N-dimethylformamide (DMF) and then exchanged with solvents such as acetone,5b absolute ethanol,7a-f or chloroform followed by hexane.7g After solvent exchange, the solvated crystals are transferred into a critical point dryer and treated through several cycles of soaking in liquid CO2 and venting. After the solvent is exchanged by liquid CO2, the temperature is raised to 40° C. and the pressure to 80-200 bar followed by several hours of treatment at supercritical conditions. Release of the supercritical conditions and gradual venting of the CO2 yields the activated sample.